Diesel, among the various fuels derived from crude mineral oils, have advantages such as good fuel efficiency, low cost and low carbon dioxide generation. On the other hand, there is a problem that combustion of diesel produces a large quantity of air pollution, especially particulates matters. In order to solve the problem, various researches have been conducted on alternative fuel which has similar physical property to diesel, and is economically preferable, and also can prevent the air pollution. The bio-diesel has similar physical property to diesel oil, remarkably reduces air pollution, and is naturally recycling energy source. Generally, the bio-diesel is produced by trans-esterification reaction of vegetable oil such as rapeseed oil, soybean oil, sunflower oil, palm oil, etc, animal fats, waste cooking oil, and so on with alcohol in the presence of acid catalyst or alkali catalyst. In the production of the bio-diesel, about 10 weight % of glycerin with respect to the total amount of bio-diesel is produced as a by-product. Recently, since the plant construction for bio-diesel is rapidly and world-widely progressed, an oversupply of glycerin is expected.
On the other hand, oils and fats generally contain free fatty acids, which exist in the mixed form with triglyceride of fatty acid. The free fatty acids are separated as the by-product in the refining process of oils and fats. Several methods for preparing fatty acid alkyl ester from the separated free fatty acids have been known. The methods for esterification of the free fatty acids are disclosed in European patent publication No. 127104A, European patent publication No. 184740A and U.S. Pat. No. 4,164,506, and so on. In the methods, the esterification reaction is carried out by heating the mixture of fatty acid and fatty acid triglyceride with methanol at about 65° C. in the presence of sulfuric acid or sulfonic acid catalyst. European patent publication No. 708813A discloses the method for increasing the yield of the fatty acid alkyl ester from oils and fats. In the method, the free fatty acid is separated from glycerin phase which is the product of transesterification reaction, and then the separated free fatty acid is esterified. In this method, the free fatty acid is obtained by the neutralization of glycerin phase, and the obtained free fatty acid is reacted for 2 hours at about 85° C. in the presence of strong sulfuric acid catalyst, which reduces the amount of fatty acid from 50% to 12%. In addition, a method for improving esterification reaction efficiency of fatty acid is disclosed (Korean patent unexamined-publication No. 2004-0101446, International Publication No. WO 2003/087278), which utilizes a mechanical apparatus or supersonic waves for causing dynamic turbulence in a reactor. In this method, the esterification is carried out by reacting the fatty acid and/or fatty acid contained in oils and fats with alcohol at a high pressure and a high temperature using sulfuric acid or ion exchange resin as catalyst. Further, Korean patent unexamined-publication No. 2004-87625 discloses a method for removing free fatty acid from waste cooking oil, using solid acid catalyst. The above mentioned methods commonly use an acid catalyst, such as sulfuric acid etc. If such an acid catalyst is not completely removed after the reaction, the quality of bio-diesel is deteriorated. Therefore, complicate processes for neutralizing, filtering, washing and cleaning the acid catalyst must be needed. Also the lifecycle of the solid acid catalyst is not long and cost for recycling the same is expensive. Furthermore, in the above-mentioned conventional methods, since the esterification of fatty acid is carried out at low temperature, water produced during the reaction is not efficiently removed to outside of the reaction system. Thus, the conversion ratio of fatty acid into fatty acid alkyl ester is low, and the physical properties of the obtained fatty acid alkyl ester are not suitable for bio-diesel.